1. Field of the Invention
The present invention relates to the manufacture of alcohol. More particularly, it relates to a process for the continuous production of ethanol.
2. The Prior Art
In light of the steadily increasing demand for liquid fuels and the shrinking resources for petroleum crude oil, researchers have begun to investigate alternative liquid fuels to determine the feasibility of commercially producing such substitutes in order to fulfill this increasing demand. Recent world events, including the shortage of petroleum crude oil, the sharp increase in the cost of oil and gasoline products, and the political instability of many oil-producing countries, have demonstrated the vulnerability of the present sources of liquid fuels. Even if such supply and economic instabilities were acceptable, it is clear that the worldwide production of petroleum products at forecasted levels can neither keep pace with the increasing demand nor continue indefinitely. It is becoming evident that the time will soon come when there will have to be a transition to resources which are plentiful and preferably renewable.
One of the most generally recognized substitutes which could be made available in significant quantities in the near future is alcohol, and in particular, ethanol. See "The Report of the Alcohol Fuels Policy Review" (Dept. of Energy/PE-0012, June 1979). For example, there are currently many outlets in the United States and throughout the world which sell a blend of gasoline and about 10 percent to 20 percent ethanol (commonly called "gasohol") which can be used as a fuel in conventional automobile engines. Furthermore, ethanol can be blended with additives to produce a liquid ethanol-based fuel (that is, ethanol is the major component) which is suitable for operation in most types of engines. Such an ethanol-based fuel is disclosed in copending United States application Ser. No. 087,618 filed on Oct. 23, 1979. It is to the problem of how to produce sufficient quantities of ethanol needed for use in such substitute fuels in order to meet the increased demand for liquid fuels that the present invention is directed.
It is well-known that ethanol can be produced by fermentation. Even today, throughout most of the world, ethanol is produced through the fermentation process. In the United States, however, only about 25 percent of the total production of ethanol is by fermentation, the remaining portion being synthetically produced, generally from ethylene.
In the fermentation process, yeast is added to a solution of simple sugars. Yeast is a small microorganism which uses the sugar in the solution as food, and in doing so, expels ethanol and carbon dioxide as byproducts. The carbon dioxide comes off as a gas, bubbling up through the liquid, and the ethanol stays in solution. Unfortunately, the yeast stagnate when the concentration of the ethanol in solution approaches about 18 percent by volume, whether or not there are still fermentable sugars present.
Accordingly, in order for nearly complete fermentation, and in order to produce large quantities of ethanol, the common practice has been to use a batch process wherein extremely large fermentation vessels capable of holding upwards of 500,000 gallons are used. With such large vessels, it is economically unrealistic to provide an amount of yeast sufficient to rapidly ferment the sugar solution. Hence, conventional fermentation processes have required 72 hours and more because such time periods are required for the yeast population to build to the necessary concentration. For example, a quantity of yeast is added to the fermentation vessel. In approximately 45-60 minutes, the yeast population will have doubled; in another 45-60 minutes that new yeast population will have doubled. It takes many hours of such propogation to produce the quantity of yeast necessary to ferment such a large quantity of sugar solution.
Furtermore, the sugars used in such traditional fermentation processes had typically contained from about 6 percent to 20 percent of the larger, complex sugars (such as dextrins and dextrose) which take a much longer time to undergo fermentation, if they will undergo fermentation, than do the simple hexose sugars (such as glucose and fructose). Thus, it is common practice to terminate the fermentation process after a specified period, such as 72 hours, even though not all of the sugars have been utilized. Viewing the prior art processes from an economic standpoint, it is preferable to sacrifice the remaining unfermented sugars than to wait for the complete fermentation of all of the sugars in the batch.
In addition, experience has taught that it is preferable to add malt enzymes which aid in the hydrolysis of starches and conversion of the higher complex dextrin and dextrose sugars which are present in the sugar solutions of the prior art fermentation processes. While such malt enzymes add a desirable flavor to ethanol produced for human consumption, the malt enzymes do nothing to make ethanol a more advantageous liquid fuel substitute and, in fact, could create problems for such a use.
One of the important concerns with conventional fermentation systems is the difficulty of maintaining a sterile condition free from bacteria in the large-sized batches and with the long fermentation period. Unfortunately, the optimum atmosphere for fermentation is also extremely condusive to bacterial growth. Should a batch become contaminated, not only must the yeast and sugar solution be discarded, but the entire fermentation vessel must be emptied, cleaned, and sterilized. Such an occurrance is both time-consuming and very costly.
After fermentation, traditional processes have removed the ethanol from the fermentation solution and further concentrated the ethanol product by distillation. Distillation towers capable of such separation and concentration are well-known in the art.
From the foregoing, it is clear that the form of the sugars used in the fermentation process is important to the efficiency of production and the yield of ethanol. It is highly desirable that sugars used in the fermentation process preferably be the simple hexose sugars so that the fermentation period is minimized and as much as possible of the sugar can be utilized in the fermentation process, thereby resulting in a higher yield of ethanol.
Prior art ethanol fermentation processes have generally been restricted to the use of small grains as the source of the fermentable sugars. These grains are particularly advantageous because the starch therein is readily hydrolyzed to sugars. Unfortunately, while most of the resulting sugars are fermentable, typically 6 percent to 20 percent of the sugars are the slow fermenting or nonfermenting complex sugars. Moreover, to obtain the fermentable sugars from such grain sources is extremely expensive. Thus, if large quantities of ethanol are to be produced for use as a substitute liquid fuel at a reasonable cost, other sources must be considered for obtaining the fermentable sugars. Although it has been shown under laboratory conditions that such sugars can be obtained from cellulose-containing materials, the hydrolysis process for releasing the fermentable sugar is known to be very difficult. Hence, researchers in the past have not found an economically acceptable method for manufacturing ethanol from such cellulosic sources.